The present invention relates to applying heat to a moving web of material and, in particular, to an apparatus and method for preheating a paper web to precondition the same, as for combination in a corrugated paperboard web.
In the manufacture of corrugated paperboard, the various component paper webs are usually preheated prior to the application of an adhesive and the joining of one web to another. A common preheating device in a corrugator comprises a rotating steam-heated roll around which the moving paper web is partially wrapped and heated before it is transferred to downstream processing equipment, such as a single facer or a double backer.
More recently, it has been found that web preheating devices may be substantially simplified and the heat transfer to the web provided more efficiently by utilizing a heated stationary plate over which the web travels. It has also been found to be effective to apply a vacuum holddown force to such hot plates by providing vacuum openings in the heating surface to which a vacuum is applied from the underside of the surface. Although such vacuum hot plates are much simpler in construction than rotary preheating rolls, other processing problems have arisen.
Because a vacuum hot plate must be wide enough to process the widest conventional webs (e.g. 96" or about 2400 mm), when narrower paper webs are being processed, the lateral edges of the hot plate are exposed resulting in heat loss and the requirement for increased vacuum flow. In addition, the vacuum induced air flow around the edges of the web tends to provide a cooling effect resulting in uneven heating of the web.